Image intensifiers electrically amplify light reflected from a scene in a low light level scenario. They can be integrated into direct viewing systems such as periscopes, monoculars, and night vision goggles or CCD (charge coupled device) imaging systems. In these various formats, intensified imaging systems have both industrial and military applications.
Advances in the design of image intensifier tubes have produced multiple models with varying capabilities, components, and features; for example GEN 0, GEN I, GEN II and GEN III tubes. A problem common to all of the tubes is saturation and potential damage at high light level viewing. An image intensifier works in the following manner. An objective lens collects the incident radiation and focuses it onto the photocathode. The photocathode absorbs this light energy and converts it to electrons. In GEN 0 and GEN I intensifiers these electrons are accelerated towards a phosphor screen maintained at a higher potential than the photocathode. The phosphor screen converts the electron emission to visible light which is significantly amplified by the process. In GEN II and GEN III intensifiers, the electrons generated by the photocathode pass through a microchannel plate (MCP) that multiplies the number of electrons prior to their striking the phosphor screen.
A variety of control systems have been produced with the goal of controlling the brightness detected by the image intensifier tube. Some control systems are limited to a specific generation of tube to which they can be applied. However, the general approach thus far in the prior art is to monitor and control the total intensity impinging on the intensifier tube across the entire scene, and once a designated threshold limit is reached, voltage to the photocathode, and thus sensitivity and spatial resolution, is significantly reduced.
More specifically, the prior art, as described in U.S. Pat. No. 5,135,424, includes a flux monitor circuit which measures the flux of the incident light on the image intensifier tube, as well as an intensity monitor circuit described in U.S. Pat. No. 4,853,529, which measures the intensity of incident light on the image intensifier tube for a preset time period. Both circuits reduce voltage supplied to the image intensifier tube when a threshold value is reached during operation. Additional prior art described in U.S. Pat. Nos. 4,872,057 and 4,882,481 provides various means of on/off gating the duty cycle duration in relation to the incident light in order to control the light level detected by the intensified camera system. Other prior art described in U.S. Pat. No. 5,146,077, utilizes the varying photocathode current to detect excessive incident light. One invention, as described in U.S. Pat. No. 4,695,718, uses the fluctuations in the photocathode current to control a variable density filter placed in front of the photocathode, while another invention uses the varying photocathode current to adjust the photocathode voltage. In yet another prior art invention described in U.S. Pat. No. 4,433,236, the photocathode current is monitored to provide a warning to the operator by means of a blinking on the phosphor screen when a threshold level is reached.
The multitude of prior art in this area concentrates on the protection of the image intensifier tube from excessive incident light by shutting down or reducing the power to the tube for the duration of the saturation. In a variety of both military and industrial situations this procedure could have very negative effects including complete loss of night vision and loss of acquired data. In many instances the excessively bright source comprises only a fraction of the viewed scene.